setting may result in packing in the crusherchamber, high loads and stresses, and possibledamage to major components.After passing the dual roll crusher, the material isrecirculated to the top screen by means of a returnconveyer and a revolving elevator wheel. The materialfrom the hydrocone crusher is recirculated to the topscreen by means of a return conveyer and a crossoverchute. Any material still too large to pass the top screenis routed through the roll crusher or hydrocone crusheragain. This is known as a “closed circuit” crushingsystem. It is obvious that the product setting on the rollcrusher or hydrocone crusher should be set at or slightlyless than the opening size of the top screen.The material retained on the bottom screen is theproduct that is routed through a chute to a conveyer andthen delivered to the product stockpile. Material toosmall to meet product specifications is screened out bythe bottom screen and delivered to a by-productstockpile.The secondary unit includes a bar type of scalpingscreen that can be inserted into the top rim of the slopinghopper. When inserted, this screen causes any oversizedmaterial to be scalped and rolled off of the screen andto the rear of the machine.The same factors discussed for the jaw crusher alsoaffect the production of the roll crusher. By comparison,dual roll crushers have a limited “stage of reduction”capability. “Stage of reduction” is the difference,expressed in inches or centimeters, between themaximum input and maximum output size of materialdue to a single crushing action. A stage of reduction of3 inches (7.6 cm) indicates a 3-inch (7.6-cm) reductionin maximum particle size. This reduction capability is afunction of the diameter of the rolls and of the nature ofthe roll shell surface; therefore, every different size andcombination of roll shells has a somewhat differentstage of reduction capacity. For this reason themaximum size of material to be fed to a dual roll crusheris critical.The maximum allowable feed size is the sum of thestage of reduction capability and the product setting. Ifthe maximum feed size is exceeded, unsatisfactoryresults likely to occur are as follows:1. Retarded production2. Excessive roll shell wear3. Excessive long-and-flat particlesThese units also have a wide range of productioncapability. The actual rate of production in tons per houris usually determined by the screening capacity of thescreens. In some cases, the rate may be limited by thecapacity of the dual roll crusher or hydrocone crusher.ScreensCrushed rock particles are separated into two ormore particle size ranges by the use of screens (fig. 6-9).Screens are also used to scalp off oversized rock and toFigure 6-9.-Vibrating screen box.6-8
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